1. Field of the Invention
The present invention relates to an echo controlling apparatus for use in a video conferencing system. More particularly, the present invention relates to an echo controlling apparatus of a video conferencing system using a personal computer (PC) and a control method using the same.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating a general video conferencing system.
In FIG. 1, an audio signal of a person in a conference room A taking part in a video conference, generated via a first microphone 10, is transmitted to a second speaker 16 of a conference room B via a first echo controller 11, a transceiver 14, and a second echo controller 15. The audio signal output from the second speaker 16 is provided to a second microphone 18 after being echoed by a wall 17 of the conference room B or being directly coupled to the second microphone 18. If the echo controllers 11 and 15 do not operate to control echo, the audio signal output from the second speaker 16 is fed back to a first speaker 12 at the conference room A via the second microphone 18 and the transceiver 14. The feedback audio signal output from the first speaker 12 is provided to the first microphone 10 after being echoed by a wall 13 of the conference room A or being directly coupled to the first microphone 10. Thus, an echo phenomenon occurs in the conference room A, where the person who has spoken in the conference room A can hear his own voice after a time lag.
The function of the echo controllers will be described with reference to FIG. 2.
Referring to FIG. 2, a far-end input signal Y which is an audio signal transmitted from the opposing conference room is output to the speaker 12 of the conference room A, and includes an echo signal e. The output from an adaptive filter 23 is an estimated echo signal e. The echo signal e output from the speaker 12 is converted into an electrical signal via the microphone 10, and then input to a subtractor 22. Also, a voice signal of a person speaking in the conference room A to an audience in the remote conference room B, i.e., near-end input signal S.sub.0, is provided to the subtractor 22 via the microphone 10.
If the amplitude of the echo signal e input to the subtractor 22 via the microphone 10, which has been output from the speaker 12, is the same as that of the estimated echo signal e output via the adaptive filter 23, the echo signal e and the estimated echo signal e are offset, so that only the voice of the person speaking in the conference room, i.e., pure near-end input signal S.sub.0, is transmitted without any echoing phenomenon.
The estimated echo signal e output from the adaptive filter 23 is expressed by the equation e=h.times.Y, and an audio input signal S via the microphone 10 is expressed by the equation S=S.sub.0 +e. In these equations, Y represents a far-end input signal, h represents a transfer characteristic (i.e., an adaptive coefficient) of the adaptive filter, S.sub.0 represents a near-end input signal, e represents the echo signal fed back to the microphone 10 from the output of the speaker 12.
An audio signal S.sub.t transmitted to the opposite conference room is expressed by the equation S.sub.t =S-e=S.sub.0 +e-e. Echo error is defined as the difference between the estimated echo signal e and the echo signal e. If the amplitude of the estimated echo signal e is equal to that of the echo signal e input to the microphone 10, the echo error becomes zero. That is, the echo signal e is not incorporated in the audio signal S.sub.t transmitted to the remote conference room B via a transmission line, so that the audio signal S.sub.t becomes equal to the near-end input signal S.sub.0.
During such signal processing, an adaptive filter coefficient h is continuously updated by the equation EQU h(n+1)=h(n)+.alpha..times.error(n).times.Y,
where .alpha. is a constant which is set depending on the system and is usually less than 1, and h(n+1) to be updated is calculated using h(n), the constant .alpha., the previous echo error (error(n)) and the far-end input signal Y.
If the initial adaptive filter coefficient h is equal to 0, the estimated echo signal e becomes zero according to the equation e=h.times.Y. As a result, the audio signal transmitted to the opposite conference room B become the sum of S.sub.0 and e (S.sub.t =S.sub.0 +e). That is, in the initial stage, an audio signal including the echo signal e is transmitted to the opposite conference room B. Then, after a predetermined lapse of time, i.e., when the adaptive filter coefficient is not equal to zero, the audio signal S.sub.t including less of the echo signal e is transmitted according to the equation S.sub.t =S.sub.0 +e-e.
Thus, in a video conferencing system adopting such an echo controlling apparatus, an adaptation process is required for a predetermined time since the initial adaptive filter coefficient is equal to zero, which is a characteristic of the echo controlling apparatus. That is, the echo controlling apparatus is operated by simply setting the initial adaptive filter coefficient to zero, without providing appropriate parameters in advance, which are useful to properly set the initial adaptive filter coefficient.
Therefore, the adaptive filter coefficient is set regardless of the circumstances of a video conferencing system in the conference room, so that the preferred echo control cannot be performed until the echo controlling apparatus has adapted.